Patent GB-684956-A (GB)
Title: Improvements relating to rangefinders
IPC: G03B13/20
Filed: 1949-04-29
Abstract: 684,956. Optical rangefinders. PULLIN OPTICAL CO., Ltd., and BAUMGARTNER, W. J. July 6, 1950 [April 29, 1949], No. 11527/49. Classes 97 (i) and 98 (i). The rectilinear base of a coincidence rangefinder is established in two parts by prisms reflecting the image of the object, the distance of which is to be measured, from each end of the base to an eyepiece, between the ends of the base, through which the images are viewed, the two parts of the base being off-set relative to each other at right angles to the base line and to the axis of the eyepiece and, in front of one end of the base line lenses are provided, one of which is movable to bring the image passing through the lenses into register in the direction of the base line with the image reflected from the other end of the base, as seen in the eyepiece. As shown, the instrument comprises a casing 2 having entrance windows 3, 4 and a viewing aperture 5. Light from the window 4 reaches the aperture 5 via a rhombic prism 6, Fig. 2, pressed by a spring 8 against stops in the base of the casing and by a spring 10 against side stops. Light from the window 3 after passing the adjustment lenses 35, 38 is reflected to the aperture 5 by a similar prism 14 resting at one end upon the prism 6, supported at the other by levelling screws 17, 18 and similarly pressed against the base by a spring 19 and against side stops 23 and an end stop 24 by a spring 21. Adjustment of the screws 17, 18 brings the two fields of view into horizontal register and a mask 68 behind the window 4 may be adjusted horizontally to complete the registration by the insertion of an eccentric tool through a suitable aperture in the base of the instrument. Coincidence is effected by the system shown in Fig. 4 comprising a fixed planoconcave lens 35 and a plano-convex lens 38 rotatable about the common centre of curvature of the adjacent surfaces of the lenses. The lens 38 is supported on a plate 29 pivoted on a post 27 fixed to the casing and having an arm 46 held by a spring 50 against a cam 47 on a shaft 49 rotated by a knob 52, Fig. 2, the extent of rotation being limited by engagement of a projection 100 on the knob with a stop 101. The lens 35 is normally fixed, but for initial adjustment it is supported on a plate 28 also rotatable on the shaft 27 and having a slot 41 into which a tool 42 can be inserted to effect slight rotation of the plate 28 after which the latter is clamped by screws 43, 44. The knob 52, which projects slightly from the casing for ease of access, bears a range scale 57 which moves against a scale 59 indicating the depth of focus achieved by a certain photographic lens. The scale 59 is carried by a plate 58 carrying similar scales 60, 61 relating to other lenses, and rotatable by a knob 63 to bring the appropriate scale into use, the scales being viewed through an aperture 56 in the casing. In a modification, the prism 6 is replaced by two prisms pressed apart by a suitable spring, and the stop 13 is replaced by an eccentric rotation of which effects a small adjustment in the length of the rangefinder base.